1. Field of the Invention
The present invention relates to a semiconductor rectifying device used to rectify the output of A.C. generators for use with vehicles and a full-wave rectifier fabricated by using the same device.
2. Description of the Related Art
Three-phase full-wave rectifiers of A.C. generators for vehicles comprise a pair of radiators used as terminals on the D.C. side, semiconductor rectifying devices, three of which are fixedly mounted on each pair of radiators, within which the rectifying directions are aligned and between which the rectifying directions are different, and three A.C.-side terminals, which are fixedly mounted on different radiators, for connecting the semiconductor rectifying devices. Resin sealing types are used for semiconductor rectifying devices in place of conventional canned sealing types due to the advancement of technology relating to stability. Because these semiconductor rectifying devices are placed near an engine and used under severe conditions, various improvements have been made on sealing resins up to the present time. A typical example of a sealing resin comprises a combination of (1) silicone resin and (2) an epoxy resin in which a silicone resin and SiO.sub.2 powder are added, and is disclosed in Japanese Patent Laid-Open No. 59-172749 (1984).
Silicone resins are widely used as sealing resins for semiconductor rectifying devices because they have high resistance to heat and to moisture and excellent bonding properties with metals. However, since silicone resins have a large thermal expansion coefficient and a great shrinkage when cured, they are liable to peel away from metal in the bonding interface and there is a danger that defective electrical characteristics will occur in the semiconductor rectifying devices. Also, silicone resins are problematical in that they have a low resistance to salt damage. As the inventor of the present invention has confirmed, alkali is produced by electrolysis when semiconductor rectifying devices are tested with salt water. This alkali decomposes siloxane bonding, causing peeling of the bonding interface between metal and the silicone resin. This is a serious problem for semiconductor rectifying devices installed in vehicles used near the seaside or in snowy areas.
The above-described problem still exists even when a silicone resin and an epoxy resin are combined, namely, when an epoxy resin layer is laminated on a silicone resin layer. This is because the silicone resin has a thermal expansion coefficient greater than that of the epoxy resin. If a temperature cycle is added, the epoxy resin peels away from the metal on the close contact interface between them due to the expansion and contraction of the silicone resin. The bonding interfacing of the underlying silicone resin with the metal is placed under substantially the same conditions as in a single silicone resin body.
As described above, as long as well-known resins are used as sealing resins, semiconductor rectifying devices having excellent resistance to salt damage cannot be realized.